This invention relates to a porous material made up of a plurality of carbon fibers, particularly cut carbon fibers. More particularly, this invention relates to a porous carbon fiber material comprising cut carbon fibers covered with a thin metal film to form a material which may be formed into a plate or mat useful as an electrode substrate for a battery, or electrolytic electrode, a current collector, a filter material such as a magnetic filter, a mist separator, fins of a thermal radiator, and as a catalyst support, for example.
The porous material of this invention is useful, for example, as a support in the form of an electrode substrate which supports the active anodic and cathodic materials of a battery. The porous material is also useful for flow-through electrodes such as electrolytic electrodes or current collectors, and for flow-through elements such as filters including magnetic filters, mist separators and fins of thermal radiators. Moreover, it is advantageous for use in both support and flow-through elements such as supports for catalysts, for example.
The prior art discloses porous materials used for such purposes. Matted materials have heretofore been known, composed of entangled carbon fibers plated with metal. However, in such matted materials the carbon fibers are not well dispersed and so-called fiber bundles are present in which many individual fibers are aggregated or bundled together and are all oriented in the same direction, forming a unidirectional bundle. In such materials the metal is plated only upon the outer surface of the fiber bundle but not upon the carbon fiber surfaces in the interior of the bundle. Even if the metal is plated in the inner portions of the fiber bundles, it serves only to cause the individual carbon fibers to stick together, thus providing the equivalent of a very thick single fiber.
Porous materials composed of thick individual fibers have several disadvantages, as follows:
(A) The available surface area of the porous material is small, and accordingly disadvantageous in use, especially as a flow-through electrode. PA1 (B) The mesh of the porous material is so coarse that it is disadvantageous for use as a supporting device or as a flow-through element. The size of the mesh of such a porous material can be expressed in terms of the average distance between one point of fiber intersection to a neighboring point of fiber intersection. This average distance is known to be nearly proportional to the diameter of the fiber. Therefore any substantial coarsening of the fiber diameter by aggregation of single fibers is very significant since it makes the mesh of the porous material undesirably larger. The larger the mesh of the porous material, the lower its ability to maintain its shape and strength as a support, the lower its current collecting efficiency when used as a current collector, and the lower the contact probability when used as a flow-through element with a fluid. PA1 (C) It is disadvantageous to use the (thick-fiber) porous material as an electrode because of its low electrical conductivity. PA1 (D) The porosity (of a thick-fiber material) is so low that the porous material is unsuitable for both support and flow-through uses; that is, when the porosity is low, the support can offer only a small amount of active substance per unit of fiber and its weight efficiency is reduced. When used as an electrolytic electrode, activation polarization and concentration polarization become excessive, which hinders electrode performance. When used as a flow-through electrode, the fluid resistance of the plate or mat increases. PA1 (E) The (thick-fiber) porous material is not uniform, and is accordingly unsuitable for use as a flow-through element. If the porous material is not uniform, rectification of the fluid is largely disturbed. Also, it becomes difficult to attain uniform distribution of current velocity. Moreover, the fluid cannot flow smoothly, which causes an increase of fluid resistance.
For example, the outer surface area of a fiber bundle formed of one hundred individual fibers is less than one-tenth of the sum of the surface areas of one hundred independent individual fibers. On the other hand, as the aforementioned surface area becomes larger, the activation polarization and concentration polarization of an electrolytic electrode become smaller, and the collecting efficiency of the current collector becomes higher. Accordingly, the available surface area should be as great as possible. PA2 When thick fibers exist, the number of contact points with the other fibers decreases. As a result, the electrical conductivity of the porous material is lower. For use as an electrode, electrical conductivity is required to be as high as possible.
On the other hand, German Pat. No. 2,204,752 describes a porous material which is made by plating nickel on a fleece of graphite fibers, where the diameter of the graphite fiber is 100 to 400 microns and is very coarse. However, in the fleece as in the aforementioned mats, the fibers are not dispersed enough, and additionally they use such thick graphite fibers that they have all the aforementioned disadvantages (A)-(E).